Fasteners comprising a multilobular tool engaging portion are known. For instance, fasteners for use with a TORX® drive system are provided with such tool engaging portions. The tool engaging portion may be in the form of a socket formed in a fastener head, or it may be in the form of a protruding portion, extending outwardly from a fastener head. A multilobular tool engaging portion ensures that a good driving contact can be obtained between driving surfaces of a tool and driven surfaces of the tool engaging portion of the fastener, thereby ensuring efficient torque transfer between the tool and the fastener. Accordingly, the risk of the tool disengaging the fastener during driving the fastener into a workpiece is minimised.
However, in multilobular driving systems, the tool tends to ‘wobble’ or ‘wander’ when it engages a tool engaging portion of a fastener. Therefore it is often necessary to use two hands when the tool is operated. This is a disadvantage. Accordingly, it is desirable to provide a tighter fit between the tool and the fastener in order to prevent or reduce ‘wobbling’ of the tool during use, thereby allowing one-hand use of the tool.
U.S. Pat. No. 5,461,952 discloses a driver assembly for driving fasteners into a workpiece. According to one embodiment, a fastener socket is provided with a multilobular tool engaging portion where each lobe includes a driven side wall and a trailing tapered side wall. The driven side walls are adapted to contact the complementary lobular portions of a tool in order to apply torque to the fastener, while the trailing tapered side walls are aligned to contact the complementary lobular portion in wedge engagement. This reduces the wobble or walking of self-tapping screws.
One disadvantage of the driver assembly of U.S. Pat. No. 5,461,952 is that the trailing tapered side walls have the effect that the diameter of the socket increased along a direction from the base of the socket towards the mouth of the socket. Since it is desirable to ensure that the wedge engagement between the socket and the tool takes place at a certain depth of the socket, the mouth of the socket therefore has a relatively large diameter. This has the consequence that a relatively large gap exists between the socket and the tool at the mouth of the socket, and this increases the risk of instability or wobble of a tool engaging the socket of the fastener.